Soil CO2 efflux in a degraded raised bog is regulated by water table depth rather than recent plant assimilate.

Understanding the climatic and biological factors that regulate soil carbon dioxide (CO₂) efflux is crucial in peatlands because they contain a large proportion of terrestrial carbon (C). We predicted that rainfall reduction would increase soil CO₂ efflux, and that cessation of below-ground allocation of recent plant assimilate would reduce soil CO₂ efflux. These predictions were tested in the field using rainfall shelters that allowed a maximum of 40 % of rainfall onto 2 x 2 m plots by diverting rainwater from the shelter roofs with guttering, and by girdling stems of the dominant plant, Calluna vulgaris, for two years. We also used ¹³CO₂-pulse labelling of intact monoliths at ambient CO₂ concentrations to trace recent assimilate from plant shoots to roots, bulk soil, leachate, dissolved organic carbon (DOC) and soil CO₂ efflux . Soil CO₂ efflux in the sheltered plots increased in Year 1 but not in Year 2, and we found a positive relationship between soil CO₂ efflux and water table depth. Our data indicate that lowering the water table below a critical threshold (15-20 cm) affects soil CO₂ efflux. Girdling of C. vulgaris shoots resulted in no measurable reduction in soil CO₂ efflux, while only ~3 % of ¹³C fixed by shoots was recovered in soil CO₂ efflux and DOC in the 20 days after labelling. Our findings show that below-ground allocation of recent assimilate from C. vulgaris plants > 6 years old has little impact on soil CO₂ efflux. [ABSTRACT FROM AUTHOR]